Electrically small antennas are known for having narrow bandwidths. Based on the fundamental limits of antennas, the quality factor of a small antenna, which is related to its bandwidth, has a minimum tied to its electrical size. The minimum quality factor, Q, of an antenna cannot be smaller than 1/(ka)3 where the antenna is enclosed by a sphere of radius “α” and the operating frequency is the speed of light multiplied by k divided by 2π. In this equation, k represents the wave number. Q can be related to the bandwidth for any narrowband component as the ratio of the center frequency divided by 3 dB bandwidth. The lower the Q, the wider the rational bandwidth.
A high quality factor, which is equivalent to a narrow bandwidth, means the device or the antenna does not permit fast variation. This means that the rate of information passed through the device (radiated by the antenna) is limited. Based on the Linear Time-Invariant (LTI) for systems and circuits, there is no way to radiate a high data rate signal from an antenna with a small electrical size.
In general, a small antenna is also considered to be a leaky LC resonator which means that the antenna behavior is severely affected by its resonance nature. The ohmic loss associated with both a less than ideal inductor and capacitor can be added to the radiation resistance due to leakage. The impurity of L and C can also be considered in the efficiency calculation. There are three types of small antennas: (1) lumped capacitor connected to a leaky inductor (equivalent to a magnetic dipole); (2) lumped inductor connected to a leaky capacitor (equivalent to an electric dipole) and (3) leaky inductor connected to a leaky capacitor (a combination of electric and magnetic dipoles)
The type 1 antennas are a magnetic antenna and has dominant magnetic field and stored magnetic energy in its near zone. The type 2 antenna an electric antenna and has dominant electric field and stored electric energy in its near zone. A type 3 antenna is a combination of types 1 and 2 and the near zone field can be any combination of electric and magnetic fields.
Thus, there is a need to design an antenna that exceeds theoretical limits.